The present invention relates to an electric apparatus, more particularly to a pressure transducer apparatus having an impedance bridge of semiconductor strain gauge.
As pressure transducers, various transducer apparatus utilizing semiconductor strain gauges have been proposed and employed in the past because of a relatively high sensitivity to strain or a high gauge factor. In a conventional strain gauge transducer, for example, an impedance bridge and transmitter-receiver apparatus are provided, wherein the bridge is mainly composed of four semiconductor pressure sensing elements or strain gauges and a differential amplifier in the transmitter-receiver apparatus is coupled to the bridge. In the connection between the impedance bridge and the transmitter-receiver apparatus, however, the input impedance of the differential amplifier must be selected at a value much greater than that of the impedance bridge including the input resistors of the differential amplifier so that the mutual reaction should be removed.
In U.S. Pat. No. 3,836,796, there is disclosed a semiconductor pressure transducer having an impedance bridge of piezoresistive elements, wherein the bridge comprises four equal value diffused resistors formed on a thin wall semiconductor diaphragm, and to the bridge a series circuit of an operational amplifier and a buffer amplifier is coupled. And the operational amplifier raises the output signal received from the bridge via the buffer amplifier to the desired output level.
In the circuit mentioned above, however, the buffer amplifier which is provided for isolation of the bridge from the impedance variation in subsequent stage causes disadvantages. That is, such apparatus has complex construction, and therefore increases the production cost as well as the process of manufacturing. Further, the reliability of the apparatus is relatively low as the result of the error factor of the buffer amplifier.
The semiconductor strain gauges, as mentioned above, have relatively high sensitivity to strain, and are temperature sensitive. In U.S. Pat. No. 3,654,545, an impedance bridge is further provided with temperature compensating means composed of two thermistors, each of which is connected in parallel to slidewires in respective arms of the impedance bridge. Such temperature compensating means of the thermistors, however, increases expense of fabrication.
Other relevant prior arts are as follows:
(1) U.S. Pat. No. 3,841,150, issued on Oct. 15, 1974, "STRAIN GAUGE TRANSDUCER SIGNAL CONDITIONING CIRCUITRY;" PA1 (2) U.S. Pat. No. 3,457,493, issued on July 22, 1969, "MULTIPLE CONSTANT CURRENT SUPPLY;" and PA1 (3) U.S. Pat. No. 3,956,927, issued May 18, 1976, "STRAIN GAUGE TRANSDUCER APPARATUS."